Re: Engine won't go past 30MPH
To start, welcome to the forum.
50 MPH is not unattainable. It has been done but that is a high expectation for a Chinese 2 stroke engine. I feel your "builder" might have misled you to some dgree.
Your anticipated speed however doesn't concern me as much as I have to wonder, hope, that the bicycle was built to withstand the stress of it. Building an engine that will produce that kind of power and installing it on a bicycle is only half of the job. The bike needs to have brakes, bearings, spokes, rims and tires capable of sustaining what is essentially highway speeds.
Do some reading here and I believe you'll find that 30 MPH is considered the high end of what people expect from a motorized bicycle. They are not motorcycles and expecting a bicycle to perform like one is asking a lot.
It appears that you did not build this bike so I would highly suggest that you take a close look at what you're riding and make sure the items I mentioned above are up to the job you're asking of them.
Above all, be careful and ride safe.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"